Resistive earthing of electrical distribution systems in ships and similar installations

The nature and causes of overvoltages in a 3 phase distribution system are discussed. By considering the circuit created by a phase-to-earth fault it is found that there is a critical value of earth-leakage resistance which affords maximum protection against overvoltages. The critical value of this resistance is such that the current passed by it is equal to the current passed by the capacitance to earth at normal voltage and frequency. The paper describes a method of earthing the system in order to present this value of critical resistance to the fault circuit, and its effectiveness is illustrated for various types of fault. Also explained is a method devised to determine the effective system capacitance while the system is energised.